diff-lib.c: don't strdup twice

diff-lib.c: don't strdup twice

The static function read_directory in diff-lib.c is only ever called
with struct path_list lists with .strdup_paths turned on, i.e.
path_list_insert will strdup the paths for us (again).  Let's take
advantage of that and stop doing it twice.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>